Waveguide to coaxial l transition having the coaxial outer conductor extending into the waveguide



May 4, 1965 J. c. BORGHETTI 3,182,272- WAVEGUIDE TO COAXIAL L TRANSITION HAVING THE COAXIAL OUTER CONDUCTOR EXTENDING INTO THE WAVEGUIDE a mm 6 'a mum INVENTOR.

JOSEPH QBORGHETTI BY wm mzaw, G 2 OW a PM LL ATTORNEYS May 4, 1965 J. c. BORGHETTI 3,132,272

WAVEGUIDE TO COAXIAL L TRANSITION HAVING THE COAXIAL OUTER CONDUCTOR EXTENDING INTO THE WAVEGUIDE Filed April 22, 1963 2 Sheets-Sheet 2 'HIIIiIIIIIII 154 INVENTOR. JOSEPH C. BORGHETTI ATTORNEYS United States Patent 3,182,272 WAVEGUIDE T0 COAXIAL L TRANSITION HAV- ING THE COAXIAL OUTER CONDUCTOR EX- TENDING INTO THE WAVEGUIDE Joseph C. Borghetti, Marlboro, Mass, assignor to Microwave Development Laboratories, Inc, Weliesley, Mass, a corporation of Massachusetts Filed Apr. 22, 1963, Ser. No. 274,421 3 Claims. (Cl. 3332l) This invention relates to a device providing a transition between two different types of wave transmission apparatus and particularly pertains to a transitional device for coupling high frequency electromagnetic waves between a coaxial line and a waveguide.

The principal object of the invention is to provide an improved device capable of accommodating high power and transferring a broad frequency band of electromagnetic wave energy between a coaxial line and a waveguide with minim-um reflection of the incident wave energy. The problem generally associated with such a device is to provide a transition between the dominant coaxial TEM- mode and the dominant TE mode in a rectangular waveguide. The problem is somewhat more complex where the transitional device is required to change the direction of transmission of the wave energy.

In microwave transmission lines it is frequently desirable or necessary to change the transmission medium from waveguide to coaxial line or the reverse, that is, to change from coaxial line to waveguide. Many components, for example, duplexers and antenna feed horns are more easily constructed of waveguide whereas the microwave generator may be a magnetron or klystron having its output delivered through a coaxial line. To use those different transmission media in the same system, it is necessary to convert from the coaxial line to waveguide. It may also be necessary to place the coaxial line normal to the waveguide where the transmission of wave energy around a sharp corner is required. A common use of a transitional device occurs at the longer wavelengths, where it is convenient to produce a symmetrical field by use of the coaxial mode; the symmetrical field permits the transition device to be constructed as a rotary joint. That aspect of transition devices is more fully treated in Us. Patent No. 2,812,503.

The invention resides in a device providing a transition from rectangular Waveguide to a coaxial line arranged perpendicularly to the waveguide. Such devices are commonly termed L transitions. An L transition embodying the principles of the invention may be constructed as a rotary joint or may be constructed simply a a rigid mechanism having no moving parts.

The invention, in essence, resides in providing in the transition impedance matching elements which are relatively easyto manufacture because there is an absence of complex surfaces whose profiles are difiicult to duplicate. An important aspect of the invention is the use of the outer conductor of the coaxial line to form part of the impedance matching structure of the transition. The outer conductor of the coaxial line extends into the'interior of the rectangular waveguide and in conjunction with a cone terminating the inner conductor of the coaxial line, forms a gradually constricting throat from the waveguide into the coaxial line. Because the invention does not employ any elongated impedance matching structure in the waveguide, such as is shown in US. Patent No. 2,812,503, the novel transition dcvice can be more compact than prior devices. A portion of the cone is surrounded by a semicircular member which provides a short circuit at the rear of the rectangular waveguide while permitting the constricting throat to extend completely around the cone. When embodied as a rotary joint, the performance characteristics of the invention with respect to bandwidth and voltage standing Wave ratio equal or exceed the performance characteristics set forth in U.S. Patent No. 2,812,503.

The invention can be better understood by a perusal of the following detailed description which is to be read in conjunction with the accompanying drawings in which:

FIG. 1 is a longitudinal section through a device constructed in accordance with the invention;

FIG. 2 depicts a front elevational view of an embodiment of the invention, the view of FIG. 1 being taken along the section line 11;

FIG. 3 is a vie-w taken along the section line 33 of FIG. 1;

FIG. 4 is a sectional view of the cone employed in the invention;

FIG. 5 depicts the short circuiting plug terminating the Waveguide;

FIG. 5 illustrates the cone and short circuiting plug in assembled relation; and

FIG. 7 illustrates the door-knob construction of the prior art.

In transmission lines used at very high and microwave frequencies, transition devices are employed to work coaxial lines into waveguide because waveguide is a more eflicent transmission medium. Generally, it is desired that the transition device match the characteristic impedance of the coaxial line to the waveguide over the useful bandwidth of the transmission system while minimizing discontinuities arising from the geometry of the transition device. Essentially, the transition device is required to keep the voltage standing wave ratio (V.S.W.R.) at a constant minimum over a specified bandwidth in both the waveguide and in the coaxial line.

In the past, most of the commercially feasible transitions have served adequately in situations requiring a comparatively narrow bandwidth in the order, for eX- ample, of ten percent (10%). Considerable effort has been expended in creating, in recent years, transition devices whose useful bandwidth is more compatible with the useful bandwidth of the waveguide employed in the system. The useful bandwidth of waveguide, generally, falls within the 25 to 40% range.

A common technique in designing broadband transition devices is to cause a deliberate mismatch over a selected band of frequencies and employ an iris to neutralize the mismatch over a relatively wide frequency band. Currently, one design used in a transition device is the doorknob construction in which the inner conductor of the coaxial line terminates on a door-knob structure inside the waveguide. The door-knob construction is shown in FIG. 7 together with a matching iris which increases the bandwidth of the transition. That structure exhibits good matching characteristics over a broad frequency band and is capable of accommodating high power. A serious defect of the door-knob construction, however, is the complex shape of the door-knob which requires extremely high precision in manufacture and assembly.

The position of the matching iris is critical and usually has to be individually adjusted for each door-knob transition. Moreover, the contour of the door-knob is difficult to reproduce in commercial quantities, and the complex shape of the door-knob requires special equipment to check the accuracy of the contour, making door-knob transition devices expensive to fabricate.

Referring now to FIGS. 1 and 2, there is shown a preferred embodiment of the invention employing a hollow rectangular section of waveguide 1. Secured to one of the broad walls in the interior of the waveguide is a cone 2 having a centralaperture permitting the passage of the center conductor of a coaxial line into a socket 3 in choke plate 4. The member 2, shown in FIG. 4, is

essentially a truncated right circular cone terminating at its base in a right circular cylinder. Choke plate 4 in conjunction with the cone 2 forms a quarter wave choke joint around the inner conductor of the coaxial line and prevents the escape of wave energy. The inner conductor of the coaxial line, alternatively can be brazed or soldered to the cone, and where a good electrical connection is established the quarter wave choke can be eliminated. This alternative requires the coaxial line to be united with the transition so that the two cannot be separated. Where a separable connection is required, the choke plate, with its socket, permits the inner conductor to be withdrawn. Concentric iwth the cone 2, as shown in FIG. 1, is a member which is, in effect, the outer conductor of the coaxial line. The member 5 is inserted into a hole in the upper broad wall of waveguide 1 and is provided with a flange 6 which limits the depth of insertion. To insure concentricity of the cone 2 and member 5, a circular bore is made in the inner lower broad wall into which the base of cone 3 is inserted and prevented from being displaced by screws 7 and 8.

The member 5 fits tightly within the hole provided for it in the broad wall of the waveguide and the flange 6 is brazed or otherwise secured to the waveguide. The diameter of the lower end of member 5 is greater than the width of the waveguide and peripheral portions of member 5 are cut away to permit the member to be inserted into the Waveguide, as shown in FIG. 3. The lower end of member 5 protrudes into the interior of the waveguide and extends completely across it from one narrow wall to the other as is evident from FIGS. 2 and 3. A central aperture having a diameter k extends through the member 5. The diameter k is equal to the inner diameter of the outer conductor of the coaxial line which is connected to the transition device and, in effect, the member 5 is an extension of tht outer coaxial conductor. The inner conductor of the coaxial line extends through the central aperture into the socket of choke plate 4.

The lower end of the central aperture in member 5 flares outwardly and appears as the lobe 9 in FIG. 1. The flared end is produced by rounding off the inner lower edge of member 5 to form a quadrantal are as indicated by the radius p. It is to be understood that the lobe is actually annular in form. The lobe 9 and the sloping surface of cone 2 provide a flared throat having its most constricted dimension A adjacent the upper end of the cone.

The end of waveguide 1 opposite port 10 is terminated by the short-circuiting plug 11 depicted in FIG. 4. That plug is a metallic element which fits partially around the base of cone 2. A top view of the cone and its partial encirclernent by the member 11 is depicted in FIG. 6. Face 12 of member 11 is obtained by the development of a quadrant of circle. As shown in FIG. 6, the member 11 extends slightly less than halfway around the base of cone 2. The member 11 is provided with a plateau 13 on which rests the lower end of coaxial member 5, as indicated in FIG. 1.

Short circuiting plug 11 fits into the end of the waveguide so that it abuts the base of cone 2 and is secured in place by any suitable means, such as screw 14.

It should be observed from FIG. 1 that the exposed edges of the internal members have been rounded off. This is done to prevent electrical breakdown within the transition due to the concentration of the electric field at sharp corners. The device is constructed of members having regular geometric shapes such as cones and circular arcs and hence those members are relatively easy to duplicate with a high degree of accuracy and can be checked for dimensional accuracy with standard measuring instruments.

The dimensions of the transition device are critical, if the device is to perform properly at the intended frequency of operation. It is possible, by adhering to certain principles of similarity, to make use of the dimensions of an existing device in the design of a like device to operate in a different frequency band. For the principles involved, the reader is referred to pages 87 to 90 in volume 3 of the Radiation Laboratory series, entitled Microwave Transmission Circuits, published by McGraw-Hill.

The dimensions given below (in inches) are for a Waveguide to coaxial transition intended to operate in the frequency range of 8.5 to 9.6 kilomegacycles.

(1:1.122 (internal waveguide width) b: .497 (internal waveguide height) c=1.473 d: .426 e: .344

q: r=1.62 dia. s=1.480 dia. t:1.l22 dia. It: .6875 dia. v: .045 0=2715' While a preferred embodiment of the invention has been illustrated in the drawings, it is evident that modifications can be made which do not depart from the essence of the invention. It is intended therefore that the invention not be limited to the precise structure illustrated, but rather that the scope of the invention be delimited by the appended claims.

I claim:

1. In a Waveguide to coaxial L transition of the type comprising a rectangular waveguide having one end terminated in a short circuit, the rectangular waveguide having an aperture in one of its broad walls, the improvement of an impedance matching structure comprising:

an outer coaxial member having therethrough a central aperture terminating in a flared opening providing an annular lobe, the outer coaxial member being secured to the waveguide and having its annular lobed portion extending through the apertured broad Wall into the interior of the rectangular waveguide,

and a conical member in the interior of the Waveguide,

the conical member being concentric with the annular lobe and secured to the opposite broad wall,

the conical member and the annular lobe providing a gradually constricting throat from the waveguide into the central aperture of the outer coaxial member.

2. In a waveguide to coaxial L transition of the type comprising a rectangular waveguide having one end terminated in a short circuit, the rectangular waveguide having an aperture in one of its broad walls, the improvement of an impedance matching structure comprising:

an outer coaxial member having therethrough a central aperture terminating in a flared opening providing an annular lobe, the outer coaxial member being secured to the waveguide and having its annular lobed portion extending through the apertured broad wall into the interior of the rectangular waveguide,

a truncated cone in the interior of the waveguide, the truncated cone being concentric with the annular lobe and having its base secured to the opposite broad wall, the truncated cone and the annular lobe providing a gradually constricting throat from the waveguide into the central aperture of the outer coaxial member,

and a plug short circuiting one end of the waveguide,

the plug being contiguous to and extending substantially half-way around the cone, the plug being shaped to permit the throat to extend completely around the cone.

3. The improvement of an impedance matching structure according to claim 2, wherein the cone has a central aperture for permitting the passage of the central condoctor of a coaxial line, and further comprising choke means for terminating the coaxial lines central conductor.

References Cited by the Examiner 5 UNITED STATES PATENTS 2,812,503 11/57 Riblet et a1. 33398 3,023,382 2/62 Borghetti 333-98 3,086,181 4/63 Lind 33398 10 HERMAN KARL SAALBACH, Primary Examiner. 

1. IN A WAVEGUIDE TO COAXIAL L TRANSITION OF THE TYPE COMPRISING A RECTANGULAR WAVEGUIDE HAVING ONE END TERMINATED IN A SHORT CIRCUIT, THE RECTANGULAR WAVEGUIDE HAVING AN APERTURE IN ONE OF ITS BOARD WALLS, THE IMPROVEMENT OF AN IMPEDANCE MATCHING STRUCTURE COMPRISING: AN OUTER COAXIAL MEMBER HAVING THERETHROUGH A CENTRAL APERTURE TERMINATING IN A FLARED OPENING PROVIDING AN ANNULAR LOBE, THE OUTER COAXIAL MEMBER BEING SECURED TO THE WAVEGUIDE AND HAVING ITS ANNULAR LOBED PORTION EXTENDING THROUGH THE APERTURED BROAD WALL INTO THE INTERIOR OF THE RECTANGULAR WAVEGUIDE, AND A CONICAL MEMBER IN THE INTERIOR OF THE WAVEGUIDE, THE CONICAL MEMBER BEING CONCENTRIC WITH THE ANNULAR LOBE AND SECURED TO THE OPPOSITE BROAD WALL, THE CONICAL MEMBER AND THE ANNULAR LOBE PROVIDING A GRADUALLY CONSTRICTING THROAT FROM THE WAVEGUIDE INTO THE CENTRAL APERTURE OF THE OUTER COAXIAL MEMBER. 